DESCRIPTION: Plasminogen activation provides a general mechanism for producing pericellular proteolysis and it is a prominent component of many physiological and pathological processes. Despite its physiological and pharmacological importance, detailed biochemical and molecular information about plasminogen activation is sparse. The ultimate aim of this project is the acquisition of structural and biochemical data which might provide a basis for experimental design of small molecules capable of modulating the activity of several components of the fibrinolytic system, namely, plasminogen activators, plasminogen and plasmin. A combination of procedures is being applied to obtain the desired information: these include protein engineering, site directed proteolysis, X-ray crystallography and model building. The forthcoming grant period will emphasize crystallography and model building to analyze the structures of: i) a group of microplasminogen mutants designed to elucidate the mechanism of proenzyme activation and the basis of the substrate specificity of the plasminogen activator urokinase; ii) a complex of microplasminogen and the bacterial activator streptokinase; iii) a complex of microplasmin and the plasma inhibitor alpha 2-antiplasmin; and iv) native Glu-plasminogen. It has recently been found that microplasminogen and microplasmin can be cleaved chemically to yield separable fragments from which the (pro)enzyme can be reconstituted with all properties intact. This reassociation of fragments provides a convenient experimental opportunity for identifying some of the determinants of specificity in microplasminogen/microplasmin, as well as structural elements responsible for the stability of proteins, and these observations will therefore be extended.